1. Field of the Invention
The invention relates to a capacitance measuring circuit where the value of a capacitance to be measured is represented by a frequency output, which is an inverse function of the value of the sensor capacitance.
More specifically the present invention relates to a capacitance measuring circuit, comprising an oscillator circuit. A sensor capacitance forms a link of two or more series connected impedances, which is connected to the capacitance measuring circuit only at end terminals of the series connection. A terminal of the sensor capacitance is an end terminal of the series connection, which is connected to a constant voltage and the other end terminal of the series connection is connected to a sensing node, which oscillates with a square wave voltage to maintain constant voltage levels during each of the half-periods of the oscillation, and which simultaneously senses charging current flowing into the series connected impedances to enable triggering of the capacitance measuring circuit at the moments when the charging current thus sensed has changed to predetermined levels.
2. Description of the Related Art
For many years, capacitances have been measured by a range of well known standard RC oscillator circuits, where the frequency is determined by measuring the charging voltage directly on the sensor capacitance and comparing the measured charging voltage with reference voltages.
This measurement is a high impedance measurement and standard RC oscillator circuits are sensitive to stray capacitances, as these will appear in parallel with the sensor capacitance.
Changes in these capacitances are therefore impossible to discern from each other. The stray capacitances in the standard RC oscillator circuits comes mainly from the wires connecting the sensor capacitance to the oscillator circuit, and from the input capacitance of the oscillator circuit itself. The stray capacitances often are of the same magnitude as the sensor capacitance, and are not constant. Consequently, these stray capacitances present a serious problem in practice.
U.S. Pat. No. 4,737,706 discloses a capacitance measuring circuit in which the sensor capacitance forms a link of two or more series connected impedances. Here, current sensing and square wave generation is performed by an operational amplifier with a feed-back resistor. The output voltage of the operational amplifier, which is a function of the charging current in the series connected impedances, is compared in a voltage comparator with a positive and a negative reference voltage during the positive and the negative cycles of the oscillator, respectively.
As explained, this circuit has the advantage of reducing errors coming from the stray capacitance of the sensing node and the stray capacitance of the connection to the sensor capacitance.
This capacitance measuring circuit functions by measuring the charging current of the sensor capacitance, through the series connected impedances, connected to the low impedance sensing node of the measuring circuit.
The high impedance of the sensor capacitance is thus insulated from the low impedance sensing node by the series connected impedances. As a result, the influence of the stray capacitance at the sensing node and the influence of the stray capacitance of the connecting node between the sensor capacitance and the series connected impedance are separated.
By placing the series connected impedance directly at the sensor capacitor, the stray capacitance at the connecting node between the sensor capacitor and the series connected impedance may be very small, and not of any importance.
The low impedance of the sensing node will rapidly charge the stray capacitance of the sensing node, and long before the trigger level of the charging current is reached and therefore the influence of the stray capacitance of the sensing node is reduced.
In practice the capacitance measuring circuit disclosed in U.S. Pat. No. 4,737,706 has a number of limitations, primarily because the ability of the circuit to reduce the influence of stray capacitances depends on a fast and very low impedance source to generate the square wave voltage, which is impressed on the series connected impedances. In addition, the square wave voltage, as generated in U.S. Pat. No. 4,737,706 by the output of an operational amplifier, has relatively long rise times and a relatively high source impedance.
Therefore stray capacitances are only reduced to a certain degree.
Secondly the precision of the circuit relies mainly on the speed and the precision of the measurement of the charging current and its conversion into a voltage.
The input bias currents and offset voltages of the operational amplifier in the circuit disclosed in U.S. Pat. No. 4,737,706 are temperature dependent and will influence the measurement of the charging current into the series connected impedances. Temperature dependent variations in the open loop amplification factor of the operational amplifier will influence the conversion of the charging current into the output voltage presented to the voltage comparator. Lastly, the speed of the available operational amplifiers is limited because the output value is a voltage.
For these reasons the accuracy of the circuit disclosed in U.S. Pat. No. 4,737,706 is not ideal.
In patent No. EP 1 386 173, an accurate and fast square wave voltage is impressed on that end of the series connected impedances, which is connected to the sensing node of the capacitance measuring circuit, by shifting the complete capacitance measuring circuit alternately between two voltages with constant voltage levels in the half periods of the square wave. The square wave will be defined by these two voltage levels and if fast, low impedance switches and low impedance voltage sources are implemented and if the current comparator is fast and has a low input impedance, then this circuit generates a far more accurate square wave on the sensing node than the circuit of U.S. Pat. No. 4,737,706, and an important condition for reducing the influence of stray capacitances is fulfilled.
The capacitance measuring circuit of EP 1 386 173 has, however, the limitation that a rather high supply voltage is necessary because, in addition to the supply voltage for the current comparator etc., a certain voltage available over and also under this supply voltage is required to keep the supply current alive when the whole circuit is shifted up and down.
In practice, the supply voltage for the circuit is around three times the supply voltage for the comparator and the other components. At times where power consumption is a major issue, this high supply voltage is a definite drawback.
Furthermore, the shifting up and down of the whole circuit generates a substantial noise voltage that interferes with the time measuring circuits connected to the capacitance measuring circuit to provide the capacitance values.
Moreover, both the capacitance measuring circuits of U.S. Pat. No. 4,737,706 and EP 1 386 173 share the drawback that the accuracy of the square wave impressed on the series connected impedances is dependent on the characteristics of an amplifying circuit which, on one hand, has to measure a low voltage or a low current very precisely and, on the other hand, has to be very fast and provide a low input impedance to source a precise square wave.
The precision of the square wave is very important, because charging of the capacitor is determined by the integral of each of the periods of the square wave, which means that long rise and fall times will reduce the integral of the periods and hereby lengthen the periods, compared to the periods of capacitance measuring circuits with a perfect square wave. Because the rise and fall times are temperature dependent, the importance of a precise square wave is obvious.
In practice, the square waves typically have a period of 4 microseconds and rise- and fall times of typically 80 ns with the square wave generated by the operational amplifier of U.S. Pat. No. 4,737,706, and typically 20 ns with the square wave generated by the current amplifier of EP 1 386 173.
These rise- and fall times, each corresponds to 2% and respectively 0.5% of the period of 4 microseconds, and that again means that the square waves generated by the two circuits are 4% respectively 1% from being ideal.
With the ever increasing demand on accuracy an important advantage could be gained by reducing these rise times.